1. Field of the Invention
The present invention is concerned with a process for the removal of suspended ash from wastewater and the ultimate disposal of the ash.
2. Information Disclosure Statement
Powdered adsorptive materials, such as activated carbon, coke fines, diatomaceous earth, fly ash, etc., are used in wastewater treatment in a variety of ways. Addition of such material to a biological treatment system often provides enhanced treatment performance. The biological system may be aerobic, anoxic or anaerobic in nature. The most commonly used adsorbent material is powdered activated carbon and when carbon is mixed with biological solids in an aeration basin, constitutes the Powdered Activated Carbon Treatment (PACT.sup.198 ) process, examples of such process are described in U.S. Pat. Nos. 3,904,518 and 4,069,148. It is generally economical to regenerate the powdered carbon once its adsorptive properties are expended and to reuse the regenerate carbon in the treatment process. An efficient method of carbon regeneration is wet air oxidation of the mixture of biological solids and spent carbon. Wet air oxidation at temperatures of 400.degree. to 550.degree. F. (204.degree. to 288.degree. C.) and pressures of 350 psig to 1900 psig (2413 to 13,100 KPa) destroys the volatile portion of the biological solids and oxidizes the organic substances adsorbed on the surface of the powdered carbon restoring adsorptive capacity. The resulting regenerated carbon is recycled to the treatment process.
The above described carbon regeneration/biomass destruction is distinguished from the wet air oxidation of primary sludge, activated sludge, or mixtures thereof, at lower temperatures of 300.degree. to 398.degree. F. (150.degree.-200.degree. C.) and pressures of 100 to 300 psig (700 to 2070 KPa) which is termed thermal conditioning of sludges. See W. B. Gitchel U.S. Pat. Nos. 3,272,740 and 3,359,200. The purpose of thermal conditioning is to render the sludge more easily dewaterable prior to final disposal.
An alternative method of carbon regeneration and biological solids disposal is controlled flame incineration operating under limited oxygen conditions. This thermal regeneration is generally carried out under a controlled atmosphere such as in a multiple hearth furnace. The hot thermally regenerated carbon and biomass residue mixture is cooled by introduction into a water quench tank. The resulting aqueous slurry or regenerated carbon is recycled to the treatment process.
The ash content of the regenerated carbon stream from either regeneration method can increase due to inorganic contaminants removed from the wastewater by the carbon and from the residue of the oxidized biological solids. Virgin powdered activated carbon also contains varying amounts of ash (5-35%) and oxidation of small fractions of the carbon during successive regenerations can result in an increase in the proportion of ash to carbon. The recovered aqueous slurry from either regeneration method consists primarily of reactivated carbon particles and inorganic ash particles. The continued recycle of this inorganic material to the treatment process along with the recovered carbon eventually leads to an undesirable increase in the ash content of the wastewater treatment system. Thus there is a need to remove a portion of the ash from the regenerated carbon stream to prevent a buildup of inert solids which could be detrimental to operation of the treatment process.
Armold et al. U.S. Pat. No. 4,541,933 discloses the separation of ash from a mixture of carbon, ash and supernatant which results from wet oxidation of waste activated sludge/powdered carbon mixtures. The method uses a plurality of hydrocyclones to concentrate the ash followed by collection of the ash concentrate on a screen.
Japanese patent disclosure No. 96713/81 reports the separation of ash from wet air regenerated carbon by diluting the regenerated carbon/ash mixture with water, or with water and a dispersing agent to hold the ash in suspension while the carbon particles settle. The settled carbon fraction is recycled to the treatment process while the ash particles contained in the dispersed ash slurry are subsequently concentrated by addition of cationic flocculent (1-2 ppm), dewatered, and removed from the process.
The ash contained in the dispersed ash slurry is comprised of very fine particles, and when suspended by the addition of dispersing agents, can be very difficult to concentrate. The very fine particle size of the ash prevents its removal from the dispersed fraction by conventional filtration means. Likewise, the ash particles may not settle during conventional gravity settling or may require extended periods of settling time. Additionally, coagulation and sedimentation using chemical or polyelectrolyte addition may not be effective or may result in formation of a gelatinous mass of solids which is essentially impossible to dewater. Other means of dewatering the dispersed ash slurry such as filtration, centrifugation or drying beds can be ineffective or uneconomical.
The present invention provides an efficient and economical method for removal and ultimate disposal of ash particles contained in a dispersed ash slurry derived from the above described process.